This invention relates to a silicone elastomer composition which is extrudable, has improved pot life, cures to a high strength, high durometer silicone elastomer, and which has improved mold release.
Silicone elastomers can be made from many different polyorganosiloxanes, cure mechanisms and fillers. These silicone elastomers have been made from stiff gum polymers as well as water thin fluids. The curing mechanisms have ranged from organic peroxides to moisture sensitive means to radiation. A variety of fillers have also been used, such as reinforcing silica fillers and extending fillers. One unique silicone elastomeric composition can be prepared from a vinyl-containing polydiorganosiloxane, an organohydrogensilicon crosslinker, a platinum catalyst and optionally fillers. These compositions are desirable for many reasons including curing without leaving groups, curing either at room temperature or at elevated temperature, stability at room temperature storage by using a platinum catalyst inhibitor, and capable of being made from either high viscosity or low viscosity polymers.
The present invention offers a further refinement of such silicone elastomer compositions by providing a composition which is readily extrudable and moldable at low pressure, improved in pot-life, curable to elastomers with high strength and high durometer in relatively short periods of time, and which has improved mold release.
Wada et al., U.S. Pat. No. 3,671,480, describe a heat curable elastomeric silicone composition comprising a mixture of two polydiorganosiloxanes, each of which contains vinyl unsaturation, a silica filler, a polyorganohydrogensiloxane and a platinum compound. The compositions described by Wada et al. are stated to cure to high tear strength, flame retardant, heat resistant elastomers with superior compression set. These compositions are not readily extrudable but require pressing into shape under substantial pressure and then cured.
Polmanteer et al. in U.S. Pat. No. 3,697,473 describe a composition which is fluid and consists essentially of a polydiorganosiloxane having two vinyl radicals per molecule and being terminated with triorganosiloxy group, a mixture of silicone compounds containing silicon-bonded hydrogen atoms, one compound containing two silicon-bonded hydrogen atoms per molecule and another compound containing three to 10 silicon-bonded hydrogen atoms per molecule and cured with a platinum catalyst, and optionally a reinforcing silica. Although these compositions described by Polmanteer et al. are fluid, the cured elastomers have low modulus and a low durometer with high tensile strengths and elongations. Such compositions may be useful for many applications, but do not obtain the high durometers along with the high tear strengths and tensile strengths which are required in other applications.
Jeram et al., U.S. Pat. No. 3,884,866, describe an organopolysiloxane composition for low pressure injection molding and which is described as having high tensile strength, good elongation, and high tear. These compositions described by Jeram et al. comprise 20 to 90 parts of a vinyl-containing high viscosity organopolysiloxane having a viscosity of 5 Pa.multidot.s to 1,000 Pa.multidot.s, five to 40 parts of a low viscosity vinyl-containing organopolysiloxane having a viscosity of 50 mPa.multidot.s to 5 Pa.multidot.s, five to 70 parts of filler and 0.1 to 50 parts per million of a platinum catalyst. This composition is cured by mixing 100 parts of it with one to 100 parts of a hydrogen silicone composition.
Lee et al., U.S. Pat. No. 4,162,243, describe a composition curable to a silicone elastomer. The composition is prepared by mixing a triorganosiloxy endblocked polydimethylsiloxane fluid where the triorganosiloxy is dimethylvinylsiloxy or methylphenylvinylsiloxy, a reinforcing silica having a surface treated with organosiloxane groups which contain 0.05 to 0.32 percent by weight vinyl, a fluid organohydrogensiloxane, a platinum catalyst and optionally a platinum catalyst inhibitor. The polydimethylsiloxane fluid has a major peak molecular weight of 68,000 to 135,000, a dispersity index greater than 3.8, the lowest molecular weight species between 854 and 3146 and the highest molecular weight species between 174 and 370,000. The compositions are readily extrudable under low pressure and cure to high strength elastomers with a high durometer. However, such compositions'pot-life and mold release properties are not as good as desired in certain applications. The present inventors have found that by adding certain hydroxy end-terminated polydimethylsiloxanes within a defined concentration range to the compositions of Lee et al., the pot-life of the composition as well as the mold release of the cured composition can be improved.